The technical field of this invention is dielectrometry and, in particular, devices for sensing changes in the dielectric properties of materials undergoing state transitions such as resins undergoing curing.
It has become common for the manufacturers of parts molded from polymeric composites to employ on-line monitoring devices to measure the progress of curing. For example, parts of optimal density and strength can require careful control of the heat-up rate, temperature gradients within the part, the timing and amount of applied pressure and the cooling rate. In the past, control of these parameters has been conducted according to fixed schedules and often determined by trial-and-error methods.
A.C. measurements of dielectrical properties by sensors implanted within a curing polymer can provide useful data on curing and other material properties. In particular, U.S. Pat. No. 4,423,372 issued to Senturia et al in December, 1983 discloses that A.C. measurements in the frequency range of 1 Hz to 10 kHz can be reliable indicators of curing. See also U.S. Pat. No. 4,399,100 issued to Zsolnay et al in August, 1983 and U.S. Pat. No. 4,496,697 issued to Zsolnay et al in January, 1985 for further disclosures of automatic process control systems for curing polymeric materials.
Conventional sensors for measuring changes in the dielectric properties of a curing polymer are typically either formed as simple parallel plate capacitors, such as those disclosed in U.S. Pat. Nos. 4,399,100 or 4,496,697, or planar interdigitated capacitors such as those disclosed in U.S. Pat. No. 4,423,371. Unfortunately, these devices can be ill-suited for monitoring the dielectric properties of certain polymeric materials. For example, graphite-filled, polymeric resins, such as conductive polyimide and phenolic resins (e.g., PMR-15, LARC, and SKYBOND manufactured by Hexcell, U.S. Polymeric, Monsanto and others), often yield false dielectric measurements because the conductive components of the composite cause short circuits within the sensor. Conventional sensors also have a tendency to deteriorate in high temperature environments. Even when the sensor itself is not damaged by conductive particles, leakage currents can occur at high temperatures and degrade performance, thereby impairing the reproducibility and reliability of the dielectric measurements.
There exists a need for a more reliable, inexpensive, resin-cure sensor which is sensitive to dielectric property changes and which can be implanted during manufacture and be suitable for on-line testing. Preferably, the apparatus should be suitable for implantation in a wide variety of polymeric materials, including those which have conductive fillers.